Conductive paste composition containing lithium, and articles made therefrom

ABSTRACT

A lead-free paste composition contains an electrically conductive silver powder, one or more glass frits or fluxes, and a lithium compound dispersed in an organic medium. The paste is useful in forming an electrical contact on the front side of a solar cell device having an insulating layer. The lithium compound aids in establishing a low-resistance electrical contact between the front-side metallization and underlying semiconductor substrate during firing.

This application claims the benefit of U.S. Provisional Application No.61/424,259, filed Dec. 17, 2010 which is herein incorporated byreference.

FIELD OF THE INVENTION

This invention relates to lead-free paste compositions containinglithium that are suitable for fabricating electrically conductivestructures that can be used in a variety of electrical and electronicdevices including photovoltaic cells.

TECHNICAL BACKGROUND OF THE INVENTION

A conventional photovoltaic cell structure is fashioned by bringingtogether n-type and p-type semiconductors to form a p-n junction. Anegative electrode is typically located on the side of the cell that isto be exposed to a light source (the “front” side, which in the case ofa solar cell is the side exposed to sunlight), and a positive electrodeis located on the other side of the cell (the “back” side). Radiation ofan appropriate wavelength falling on a p-n junction of a semiconductorbody serves as a source of external energy to generate electron-holepairs in that body. Because of the potential difference that exists at ap-n junction, holes and electrons move across the junction in oppositedirections, giving rise to the flow of an electric current that iscapable of delivering power to an external circuit. Most industrialphotovoltaic cells, including solar cells, are provided in the form of astructure, such as one based on a doped crystalline silicon wafer, thathas been metallized, i.e., provided with electrodes in the form ofelectrically conductive metal contacts through which the generatedcurrent can flow to the external electric circuit load.

Photovoltaic cells are commonly fabricated with a front-side insulatinglayer that affords an antireflective property to the cell to maximizethe utilization of incident light. However, in this configuration, theinsulating layer normally must be removed to allow an overlaidfront-side electrode to make contact with the underlying semiconductorsurface. The front-side electrode is typically formed by firstdepositing a metal-powder-bearing, conductive paste composition in asuitable pattern by screen printing. Thereafter, the paste is fired todissolve or otherwise penetrate the insulating layer and sinter themetal powder, such that an electrical connection with the semiconductoris formed.

The ability of the paste composition to penetrate the antireflectivecoating and form a strong bond with the substrate upon firing is highlydependent on the composition of the conductive paste and firingconditions. Efficiency, a key measure of photovoltaic cell performance,is also influenced by the quality of the electrical contact made betweenthe fired conductive paste and the substrate.

Allison et al. (U.S. Pat. Nos. 5,089,172 and 5,393,558) disclose athick-film conductor composition that can be bonded to a ceramicsubstrate fashioned from aluminum nitride.

Although various methods and compositions useful in forming devices suchas photovoltaic cells are known, there nevertheless remains a need forcompositions that permit fabrication of patterned conductive structuresthat result in improved overall device electrical performance and thatfacilitate the efficient manufacture of such devices. A lead-freecomposition would be particularly desirable.

SUMMARY OF THE INVENTION

In an aspect, the present invention provides a paste compositioncomprising an inorganic solid portion comprising:

-   -   (a) about 75% to about 99% by weight based on solids of a source        of electrically conductive metal;    -   (b) about 0.1% to about 10% by weight based on solids of a glass        component comprising:        -   1-25 wt. % of SiO₂;        -   0.1-3 wt. % of Al₂O₃;        -   50-85 wt. % of at least one of bismuth oxide and bismuth            fluoride, the amount of bismuth oxide being at least 10 wt.            % and the amount of bismuth fluoride being at most 50 wt. %;            and        -   at least 1 wt. % of at least one of TiO₂, ZrO₂, Li₂O, ZnO,            P₂O₅, LiF, NaF, KF, K₂O, V₂O₅, GeO₂, CeO₂, or a mixture            thereof;        -   wherein the weight percentages are based on the total glass            component; and    -   (c) about 0.1 to about 5% by weight based on solids of a        lithium-containing additive;        wherein the inorganic solid portion is dispersed in an organic        medium and the paste composition is substantially Pb-free.

In another aspect, there is provided an article comprising:

-   -   (a) a semiconductor substrate having a first major surface; and    -   (b) a deposit of a substantially Pb-free paste composition on a        preselected portion of the first major surface of the        semiconductor substrate,    -   wherein the paste composition comprises an inorganic solid        portion dispersed in an organic medium, the inorganic solid        portion comprising:        -   (i) about 75% to about 99% by weight based on solids of a            source of an electrically conductive metal;        -   (ii) about 0.1% to about 10% by weight based on solids of a            glass component; and        -   (iii) about 0.1% to about 5% by weight of a            lithium-containing additive.

In still a further aspect, there is provided a process comprising:

-   -   (a) providing a semiconductor substrate having a first major        surface;    -   (b) applying a substantially Pb-free paste composition onto a        preselected portion of the first major surface,    -   wherein the paste composition comprises an inorganic solid        portion dispersed in an organic medium and the inorganic solid        portion comprises:        -   (i) about 75% to about 99% by weight based on solids of a            source of an electrically conductive metal;        -   (ii) about 0.1% to about 10% by weight based on solids of a            glass component; and        -   (iii) about 0.1% to about 5% by weight based on solids of a            lithium-containing additive; and    -   (c) firing the substrate and the paste composition, whereby the        organic medium of the paste composition is removed and an        electrode is formed that has electrical contact with the        semiconductor substrate.

In various embodiments, there are provided an article and a photovoltaiccell fabricated using the foregoing process.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be more fully understood and further advantages willbecome apparent when reference is made to the following detaileddescription of the preferred embodiments of the invention and theaccompanying drawing, in which:

FIGS. 1A-1F depict the successive steps of a process by which asemiconductor module may be fabricated. The module may in turn beincorporated into a photovoltaic cell. Reference numerals as used inFIG. 1 include the following:

10: p-type substrate

12: first major surface (front side) of substrate 10

14: second major surface (back side) of substrate 10

20: n-type diffusion layer

30: insulating layer

40: p+ layer

60: aluminum paste formed on back side

61: aluminum back electrode (obtained by firing back-side aluminumpaste)

70: silver or silver/aluminum paste formed on back side

71: silver or silver/aluminum back electrode (obtained by firingback-side silver paste)

500: silver paste formed on front side according to the invention

501: silver front electrode according to the invention (formed by firingfront-side silver paste)

DETAILED DESCRIPTION OF THE INVENTION

Solar-powered photovoltaic systems are considered to be environmentallybeneficial in that they reduce the need for fossil fuels. Nonetheless,most current photovoltaic systems are based upon components that containrelatively high levels of the element lead. Reduction of lead and otherheavy metals in the environment is also considered an importantenvironmental goal. To that end, photovoltaic systems that are lead-freewould be highly advantageous.

The present invention addresses the need for a process to manufacturehigh-performance semiconductor devices using conductor compositions thatdo not require lead, but still provide mechanically robust,high-conductivity electrodes. The conductive paste composition providedherein is beneficially employed in the fabrication of front-sideelectrodes of photovoltaic devices, which must make good electricalcontact despite the presence of a front-side insulating layer typicallyincluded in such devices.

In an aspect, this invention provides a paste composition thatcomprises: a functional conductive component, such as a source ofelectrically conductive metal; a substantially lead-free glasscomponent; and an organic medium. The paste composition also includes alithium-containing component, such as an additive comprising lithiumoxide, lithium hydroxide, a lithium salt of an inorganic or organicacid, or a mixture thereof. The lithium compound aids in etching aninsulating layer (also termed an insulating film) frequently used as anantireflective coating on the front surface of a semiconductor substrateand in establishing a low-resistance electrical contact between thefront-side metallization and underlying semiconductor substrate. Theinsulating layer often used as an antireflective coating is siliconnitride. The paste composition may include additional components.

The paste composition may contain in admixture an inorganic solidsportion comprising (a) about 75% to about 99% by weight of a source ofan electrically conductive metal; (b) about 0.1% to about 10% by weightof a substantially lead-free glass component; and (c) about 0.1% toabout 5% by weight, or about 0.1% to about 3% by weight, or about 0.2%to about 1% by weight, of at least one lithium-containing additive;wherein the above stated contents are based on the total weight of allthe constituents of the inorganic solid portion of the composition.

As further described below, the composition also comprises an organicmedium, which acts as a carrier for the inorganic solid portiondispersed therein. In an embodiment, the inorganic solid portion of thepaste composition comprises about 85% to about 95% by weight based onthe entire composition, the balance being the organics. In anotherembodiment, the inorganic solid portion of the paste compositioncomprises about 87% to about 93% by weight.

The paste composition described above can be used to form a conductiveelectrode employed in an electrical or electronic device such as aphotovoltaic cell or an array of such cells. Alternatively, thecomposition can be used to form conductors used in conjunction withcircuit elements in a semiconductor module that is to be incorporatedinto an electrical or electronic device. The paste composition describedherein can be termed “conductive,” meaning that an electrode structureformed on a substrate using the composition and thereafter firedexhibits an electrical conductivity sufficient for conducting electricalcurrent between devices or circuitry connected thereto.

In an embodiment, the source of electrically conductive metal providingthe functional conductive component in the present paste composition iselectrically conductive metal powder incorporated directly as part ofthe inorganic solids of the composition. In another embodiment, amixture of two or more such metals is directly incorporated.Alternatively, the electrically conductive metal may be supplied by ametal oxide or salt that decomposes upon exposure to the heat of firingto form the metal. Electrically conductive metals suitable for useinclude those that are or contain gold, silver, copper, nickel, and/orpalladium, as well as alloys and mixtures thereof. Silver is preferred.As used herein, the term “silver” is to be understood as referring toelemental silver metal, alloys of silver, and mixtures thereof, and mayfurther include silver oxide (Ag₂O) or silver salts such as AgCl, AgNO₃,AgOOCCH₃ (silver acetate), AgOOCF₃ (silver trifluoroacetate), Ag₃PO₄(silver orthophosphate), or mixtures thereof.

In an embodiment, the paste composition contains about 75 to about 99%by weight, or about 80 to about 90% by weight, of a source of anelectrically conductive metal, the weight percentages based on theinorganics portion.

The electrically conductive metal may be supplied as finely dividedparticles having any one or more of the following morphologies: a powderform, a flake form, a spherical form, a granular form, a nodular form, acrystalline form, an irregular form, or a mixture thereof. In anembodiment, the inorganic portion of the electrically conductive metalcomponent may include about 70 to about 90 wt. % metal particles andabout 1 to about 9 wt. % metal flakes, based on the total content ofinorganics. In another embodiment, the inorganic portion of the metalcomponent may include about 70 to about 90 wt. % metal flakes and about1 to about 9 wt. % of colloidal metal. In a further embodiment, theinorganic portion of the metal component may include about 60 to about90 wt. % of metal particles or metal flakes and about 0.1 to about 20wt. % of colloidal metal.

The particle size of metal used in the present paste composition is notsubject to any particular limitation. As used herein, “average particlesize” is intended to refer to “median particle size,” by which is meantthe 50% volume distribution size. Volume distribution size may bedetermined by a number of methods understood by one of skill in the art,including but not limited to laser diffraction and dispersion methodsemployed by a Microtrac particle size analyzer (Montgomeryville, Pa.).Dynamic light scattering, may also be used, as well as directmicroscopy. Instruments for such measurements are availablecommercially, e.g. the LA-910 particle size analyzer from HoribaInstruments Inc., Irvine, Calif. In various embodiments, the averageparticle size of metal particles of the present paste composition isless than 10 microns, or the average particle size is less than 5microns. The electrically conductive metal or source thereof may also beprovided in a colloidal suspension, in which case the colloidal carrierwould not be included in any calculation of weight percentages of theinorganics of which the colloidal material is part.

The electrically conductive metal used herein, particularly when inpowder form, may be coated or uncoated; for example, it may be at leastpartially coated with a surfactant. Suitable coating surfactantsinclude, for example, stearic acid, palmitic acid, a salt of stearate, asalt of palmitate, and mixtures thereof. Other surfactants that may alsobe utilized include lauric acid, oleic acid, capric acid, myristic acid,linolic acid, and mixtures thereof. Still other surfactants that mayalso be utilized include polyethylene oxide, polyethylene glycol,benzotriazole, poly(ethylene glycol)acetic acid and other similarorganic molecules. A suitable counter-ion for use in a coatingsurfactant includes without limitation hydrogen, ammonium, sodium,potassium, and mixtures thereof. When the electrically conductive metalis silver, it may be coated, for example, with a phosphorus-containingcompound.

In an embodiment, one or more surfactants may be included in the organicmedium in addition to surfactant included as a coating of conductivemetal powder used in the present paste composition.

As further described below, the electrically conductive metal can bedispersed in an organic medium that acts as a carrier for the metalphase and other constituents present in the formulation.

Another component in the present paste composition is a glass material,such as a glass frit, or a mixture of two or more glass materials. Theglass component may include, for example, substantially lead-free,non-crystalline glass materials such as glass formers, intermediateoxides, and/or modifiers. As used in the present specification and thesubjoined claims, the phrase “substantially lead-free” refers to acomposition to which no lead has been specifically added (either aselemental lead or as a lead-containing alloy, compound, or other likesubstance), and in which the amount of lead present as a trace componentor impurity is 1000 parts per million (ppm) or less. In someembodiments, the amount of lead present as a trace component or impurityis less than 500 parts per million (ppm), or less than 300 ppm, or lessthan 100 ppm. The minimization of lead in the present paste compositionfacilitates the disposal or recycling of devices constructed with thecomposition and mitigates the health hazard associated with the knowntoxicity of lead-bearing substances, such as the present composition.

Exemplary glass formers can have a high bond coordination and a smallionic size, and can form bridging covalent bonds when heated andquenched from a melt. Exemplary glass formers include without limitationSiO₂, B₂O₃, P₂O₅, V₂O₅, TeO₂, GeO₂, and the like. Intermediate oxidescan be substituted for glass formers, and exemplary intermediate oxidesinclude without limitation TiO₂, Ta₂O₅, Nb₂O₅, ZrO₂, CeO₂, Gd₂O₃, SnO₂,Al₂O₃, HfO₂, and the like. Glass modifiers typically have a more ionicnature, and may terminate bonds, or affect specific properties such asviscosity or glass wetting. Exemplary modifiers include withoutlimitation oxides such as alkali metal oxides, alkaline earth oxides,CuO, ZnO, Bi₂O₃, Ag₂O, MoO₃, WO₃, and the like. Optionally, theviscosity of a glass may be reduced by the introduction of fluorideanions. For example, fluorine may be supplied from at least one fluorideof Al, Li, Na, K, Mg, Ca, Sr, Ba, Zn, Bi, Ta, Zr, Hf, Mo, W, Gd, Ce, Ti,Mn, Sn, Ru, Co, Fe, Cu, Cr, or a mixture thereof. If present, the amountof fluoride is such that the glass component comprises at most 5 wt. %of elemental fluorine. In one particular embodiment hereof, silver oxidemay be dissolved in the glass during the glass melting/manufacturingprocess.

As used herein, the terms “glass frit” and “frit” refer to a particulateform of amorphous, solid oxide in which short-range atomic order ispreserved in the immediate vicinity of any selected atom, that is, inthe first coordination ring, but dissipates at greater atomic-leveldistances (i.e., no long range periodic order). Frit is conventionallyformed by grinding a bulk solid of the requisite composition to aparticulate state.

The glass component of the present composition may also include a fluxmaterial, which is a substance that when heated aids, induces, orotherwise actively participates in wetting, fusion, and flow. A fluxmaterial often aids the glass, for example, in bonding at an interfaceor in promoting sintering of the conductive metal. A flux may be addedto other bulk materials to provide greater flow or fusion than the bulkmaterial would itself experience at a selected temperature. A fluxmaterial may be fully amorphous, or it may exhibit some degree ofcrystallinity, such that its powder X-ray diffraction pattern mayinclude either or both of a broad amorphous halo and sharp crystallinepeaks that define characteristic interatomic distances in accordancewith Bragg's law. In addition, heating an amorphous frit or fluxmaterial may cause it to become partially or fully devitrified. A fritmaterial may have wetting, fusion, or flow properties similar to acrystalline flux material, and vice versa. A skilled person will thusrecognize that there exists a continuum between fluxes and frits.Exemplary crystalline flux materials may be an oxide or non-oxide, andmay comprise materials such as BiF₃, Bi₂O₃, or the like.

The glass material used in the present composition is believed to assistin the partial or complete penetration of oxide or nitride insulatinglayers on a silicon semiconductor wafer during firing. As describedherein, this at least partial penetration may facilitate the formationof an effective, mechanically robust electrical contact between aconductive structure printed using the present composition and theunderlying silicon semiconductor surface of a photovoltaic devicestructure.

In an embodiment, the present paste composition may contain about 0.1 toabout 10% by weight, or about 0.5 to about 8% by weight, or about 0.5 toabout 5% by weight, or about 1 to about 3% by weight, of the glasscomponent.

In a preferred embodiment, the present composition includes crystallineflux material and an amorphous frit material, for example, a fritmaterial having a glass transition temperature (T_(g)) value in therange of about 300 to 600° C.

In an embodiment, the glass component is substantially lead-free andcomprises the chemical elements silicon, aluminum, and bismuth admixedwith at least one other element beyond oxygen and fluorine. Morespecifically, the glass component comprises 1-25 wt. % of SiO₂; 0.1-3wt. % of Al₂O₃; 50-85 wt. % of at least one of Bi₂O₃, BiF₃, or a mixturethereof, with the provisos that the Bi₂O₃ content be at least 10 wt. %and the BiF₃ content be at most 50 wt. %; and at least 1 wt. % of atleast one of TiO₂, ZrO₂, Li₂O, ZnO, P₂O₅, LiF, NaF, KF, K₂O, V₂O₅, GeO₂,TeO₂, CeO₂, Gd₂O₃, or a mixture thereof, wherein the weight percentagesare based on the total glass component. In an another embodiment, theglass component comprises 8-25 wt. % of SiO₂; 0.1-3 wt. % of Al₂O₃;50-85 wt. % of at least one of Bi₂O₃, BiF₃, or a mixture thereof, withthe provisos that the Bi₂O₃ content be at least 10 wt. % and the BiF₃content be at most 50 wt. %; and at least 1 wt. % of at least one ofTiO₂, ZrO₂, Li₂O, ZnO, P₂O₅, LiF, NaF, KF, K₂O, V₂O₅, GeO₂, TeO₂, CeO₂,Gd₂O₃, or a mixture thereof, wherein the weight percentages are based onthe total glass component.

In still another embodiment, the glass component is substantiallylead-free and consists essentially of:

-   -   8-25 wt. % of SiO₂;    -   0.1-3 wt. % of Al₂O₃;    -   50-85 wt. % of at least one of Bi₂O₃, BiF₃, or a mixture        thereof, with the provisos that the Bi₂O₃ content be at least 10        wt. % and the BiF₃ content be at most 50 wt. %;    -   0-10 wt. % of B₂O₃;    -   0-5 wt. % of at least one of Li₂O, Na₂O, or K₂O;    -   0-5 wt. % of at least one of MgO, CaO, SrO, or BaO;    -   0-5 wt. % of at least one oxide of Zn, Ta, Zr, Hf, Mo, W, Gd,        Ce, Te, Ti, Mn, Sn, Ru, Co, Fe, Cu, Cr, or a mixture thereof;        and    -   0-10 wt. % of at least one fluoride of Al, Li, Na, K, Mg, Ca,        Sr, Ba, Zn, Ta, Zr, Hf, Mo, W, Gd, Ce, Ti, Mn, Sn, Ru, Co, Fe,        Cu, Cr, or a mixture thereof;    -   wherein the weight percentages are based on the total glass        component.

The various compounds in the compositions recited herein are specifiedon the basis of the most common valence state of the respective cation.However, a skilled person would recognize that some of the cations, e.g.Bi, may exist in other valence states, which may be used in suitableamounts in formulating the glass composition. Thus, Bi cations may besupplied from compounds in which the Bi can take on any of its possiblevalence states, and not just its most common trivalent state.

Glass materials, such as those having the formulations set forth above,may be used individually, or together in a blend of plural materials inwhich the proportions in each constituent are adjusted to provide thedesired performance, including the etching of any insulating layerpresent in a photovoltaic cell and the formation of a high-qualityelectrical contact, as described in more detail hereinbelow. The oxideor fluoride materials comprised in each of the one or more glassmaterials used in the glass component of the present paste compositionare melted together to form an intimate mixture prior to theirincorporation in the paste composition.

The glass material used in the present paste composition can have avariety of average particle sizes. In an embodiment, the averageparticle size can range from about 0.5 to 3.5 μm. In another embodiment,the average particle size ranges from about 0.8 to 1.2 μm. The glassmaterial can be produced by conventional glass-making techniques,including, for example, those in which ingredients are weighed and mixedin the desired proportions and heated in a platinum alloy crucible in asuitable furnace to form a melt. Heating is conducted to a temperatureof about 1000° C. to 1200° C. for a time sufficient for the melt tobecome entirely liquid and homogeneous. Thereafter, the molten glass isquenched and comminuted to provide the desired particle size. In anembodiment, the glass material is supplied as a powder with its 50%volume distribution (d₅₀) between 1 and 3 microns. Alternative synthesistechniques may also be used for making the glass components useful inthe present paste composition. These techniques include, but are notlimited to, water quenching, sol-gel, spray pyrolysis, or othersappropriate for making powder forms of glass.

The present composition further includes a discrete lithium-containingadditive substance, such as a crystalline lithium-containing compound ora lithium-containing salt, or a mixture of two or more thereof. Asuitable lithium-containing component may be in powder form, and mayinclude at least one substance such as lithium carbonate (Li₂CO₃),lithium oxide (Li₂O), lithium hydroxide (LiOH), lithium fluoride (LiF),lithium phosphate (Li₃PO₄), other lithium salts of inorganic or organicacids including lithium soaps, or any compound that can generate metaloxides of lithium during a firing process, as well as mixtures thereof.In an embodiment, the additive may also be a mixed oxide of lithium andanother metal. The lithium-containing additive comprises about 0.1% toabout 5% by weight, or about 0.1% to about 3% by weight, or about 0.2%to about 1% by weight, of the lithium-containing component, based on thesolids of the present paste composition.

The lithium-containing component, such as Li₂CO₃, may have an averageparticle size that is in the range of about 10 nanometers to about 10microns, or in the range of about 40 nanometers to about 5 microns, orin the range of about 60 nanometers to about 3 microns, or in the rangeof about 0.1 to about 1.7 microns, or in the range of about 0.3 to about1.3 microns, or that is less than 0.1 μm. In one embodiment, Li₂CO₃ ispresent in the range of 0.1 to 5% by weight based on the solids of thepaste composition. In still a further embodiment, Li₂CO₃ is present inthe range of 0.1 to 3% by weight.

While the present invention is not limited by any particular theory ofoperation, it is believed that, upon firing, the discrete lithiumcomponent acts in concert with the glass material in the present pastecomposition to promote etching and rapid digestion of the insulatinglayer conventionally used on the front side of a photovoltaic cell. Theefficient etching in turn permits the formation of a low-resistance,front-side electrical contact between the conductive metal(s) of thecomposition and the underlying substrate. Ideally, the firing processresults in a substantially complete removal of the insulating layerwithout any further combination of the metals with the underlying Sisubstrate. Although Li is known as a constituent of some oxide glasses,its separate inclusion in the present paste composition in the form ofone or more discrete Li compounds is believed to improve the kinetics ofthe etching of the insulating layer. Surprisingly, fabrication ofhigh-efficiency photovoltaic cells is possible using the present pastecomposition with its Li-containing additive.

In preparing a paste composition of this invention, the inorganiccomponents described above may be mixed with an organic medium, e.g. bymechanical mixing, to form a viscous composition referred to as a“paste”, which has suitable consistency and rheology for a printingprocess such as screen printing. The organic medium is typically avehicle in which the inorganic components are dispersible with a gooddegree of stability. In particular, the composition preferably has astability compatible not only with the requisite manufacturing,shipping, and storage, but also with conditions encountered duringdeposition, e.g. by a screen-printing process. Ideally, the rheologicalproperties of the medium are such that it lends good applicationproperties to the composition, including stable and uniform dispersionof solids, appropriate viscosity and thixotropy for screen printing,appropriate wettability of the paste solids and the substrate on whichprinting will occur, a rapid drying rate after deposition, and stablefiring properties.

A wide variety of inert viscous materials can be used in an organicmedium in the present composition including, without limitation, aninert, non-aqueous liquid that may or may not contain thickeners,stabilizers, or surfactants. By “inert” is meant a material that may beremoved by a firing operation without leaving any substantial residuethat is detrimental to final conductor line properties. The solventsmost widely used to form such a paste composition are ester alcohols andterpenes such as alpha- or beta-terpineol or mixtures thereof with othersolvents such as kerosene, dibutylphthalate, butyl carbitol, butylcarbitol acetate, hexylene glycol, and high-boiling alcohols and alcoholesters.

In another embodiment, the organic medium may be a solution of one ormore polymers, such as ethyl cellulose, in a solvent. Other examples ofsuitable polymers include ethylhydroxyethyl cellulose, wood rosin,mixtures of ethyl cellulose and phenolic resins, polymethacrylates oflower alcohols, and a monobutyl ether of polyethylene glycolmonoacetate. When a polymer is present in the organic medium, itscontent therein may be in the range of about 8 wt. % to about 11 wt. %.A composition of the present invention formed as a paste having goodwetting characteristics typically contains 85 to 95 wt. % of theinorganic components and 5 to 15 wt. % of the organic medium. In oneembodiment, the Li₂CO₃ is present in the range of 0.1 to 5% by weightbased on solids. In still a further embodiment, the Li₂CO₃ is present inthe range of 0.1 to 3%.

As a paste, the present composition can be applied on a preselectedportion of the substrate in a variety of different configurations orpatterns, such as bars or lines useful as an electrode. Alternatively,the preselected portion may cover substantially all of a major surfaceof the substrate. The electrode is formed by depositing the paste on thesubstrate in a preselected pattern, drying the paste (optionally byexposure to a modestly elevated temperature), and thereafter firing thedeposited, dried paste. The firing process removes the organic medium,sinters the conductive metal in the composition, and establisheselectrical contact between the semiconductor substrate and the firedconductive metal. The substrate may be a semiconductor such as a thinsingle-crystal or multi-crystalline silicon wafer having first andsecond major surfaces on its opposite large sides; the substrate ispreferably a junction-bearing substrate. Firing may be performed in anatmosphere composed of air, nitrogen, an inert gas, or a mixed gas ofoxygen and nitrogen.

The present paste composition can be deposited on the substrate by avariety of processes, such as printing. Exemplary printing processesinclude screen printing, plating, extrusion, inkjet, shaped, multiple,or ribbon printing. Conductors formed by printing and firing a pastesuch as that provided herein are often denominated as “thick-film”conductors, since they are ordinarily substantially thicker than tracesformed by atomistic processes, such as those used in fabricatingintegrated circuits. For example, thick-film conductors may have athickness after firing of about 1 to 100 μm. Consequently, pastecompositions that in their processed form provide conductivity and aresuitably used for printing processes are often called “thick-filmpastes” or “conductive inks.”

The present paste composition may be printed on the substrate in anyuseful pattern. If the substrate includes an insulating surface layer,the composition may be printed atop the layer. For example, theelectrode pattern used for the front side of a photovoltaic cellcommonly includes a plurality of narrow grid lines or fingers connectedto one or more bus bars. In an embodiment, the width of the lines of theconductive fingers may be 20 to 200 μm, 40 to 150 μm, or 60 to 100 μmwide and 10 to 30 μm thick and the fingers may be spaced by 2 to 3 mm oncenter. The thickness of the lines of the conductive fingers may be 5 to50 μm; 10 to 35 μm; or 15 to 30 μm. Since the features of the patternare opaque, light impinging on them cannot be converted by the cell,decreasing apparent cell efficiency. However, reducing the feature sizeof the conductors undesirably increases their electrical resistance. Thepossibility of increasing a trace's cross-sectional area by increasingits thickness is limited by what can be attained in practical printingor other deposition processes. Thus, cell designers typically must sizethe electrode features to balance the effects of active collection areaand ohmic losses. Such a pattern permits the generated current to beextracted without undue resistive loss, while minimizing the area of thefront side obscured by the metallization, which reduces the amount ofincoming light energy that cannot be converted to electrical energy.Ideally, the features of the electrode pattern should be well definedand have high electrical conductivity and low contact resistance withthe underlying structure.

After being deposited, the paste is dried, either under ambientconditions or by exposure to a modestly elevated temperature.Thereafter, the paste is fired, with the time/temperature profileconditions for the firing typically being set so as to enable asubstantially complete burnout and removal of the organic medium bindermaterials from the paste as it has dried on the substrate. Normally, thefiring entails some combination of volatilization and/or pyrolysis toremove the organic materials. In different embodiments, the burn-outtemperature may in the range between about 300° C. to about 1000° C., orabout 300° C. to about 525° C., or about 300° C. to about 650° C., orabout 650° C. to about 1000° C. The firing may be conducted using anysuitable heat source. In an embodiment, the firing is accomplished bypassing the substrate bearing the printed conductor through a beltfurnace at high transport rates, for example between about 100 to about500 centimeters per minute with resulting dwell times between about 0.05to about 5 minutes. Multiple temperature zones may be used to controlthe desired thermal profile, and the number of zones may vary, forexample, between 3 to 11 zones. The temperature of a burn-out operationconducted using a belt furnace is conventionally specified by the setpoint in the hottest zone of the furnace, but it is generally found thatthe actual peak temperature attained by the transiting substrate issomewhat lower.

In another aspect, the present invention relates to a process ofmanufacturing a device, such as an electrical, electronic,semiconductor, or photovoltaic device. An embodiment includes the stepsof:

(a) providing a semiconductor substrate having a first major surface;

(b) applying on a preselected portion of the first major surface a pastecomposition comprising a substantially lead-free inorganic solid portiondispersed in an organic medium and wherein the inorganic solid portioncomprises:

-   -   (i) about 75% to about 99% by weight based on solids of a source        of electrically conductive metal;    -   (ii) about 0.1% to about 10% by weight based on solids of a        substantially Pb-free glass component; and    -   (iii) about 0.1% to about 5% by weight based on solids of at        least one Li-containing component; and

(c) thereafter firing the semiconductor substrate and paste composition,

whereby, upon the firing, the organic medium is removed, and theelectrically conductive metal is sintered and an electrode is formedthat has electrical contact with the underlying semiconductor substrate.

After being deposited, the paste is preferably first dried, optionallyby exposure to a modestly elevated temperature. The time/temperatureprofile conditions for the firing are typically set so as to effectsubstantially complete removal of the organic medium.

The semiconductor substrate used in the foregoing process may include aninsulating layer on its first major surface, in which case the pastecomposition is applied over the insulating layer and the firing steppreferably acts to remove at least a portion of the insulating layer topermit establishment of contact between the metal contained in thedeposited paste composition and the underlying substrate.

Embodiments of the present method that employ a semiconductor substrateoptionally include the further step of forming the insulating layer onthe semiconductor substrate prior to the application of the pastecomposition. The insulating layer may comprise one or more componentsselected from aluminum oxide, titanium oxide, silicon nitride, SiN_(x):H(silicon nitride containing hydrogen for passivation during subsequentfiring processing), silicon oxide, and silicon oxide/titanium oxide, andmay be in the form of a single layer or multiple layers. The insulatinglayer included in some implementations provides the cell with anantireflective property, which lowers the cell's surface reflectance oflight incident thereon, thereby improving the cell's utilization ofincident light and increasing the electrical current it can generate.The thickness of the layer is preferably chosen to maximize theantireflective property in accordance with the layer material'srefractive index. In some embodiments, the deposition processingconditions are adjusted to vary the stoichiometry of the layer, therebyaltering properties such as the refractive index to a desired value. Fora silicon nitride film with a refractive index of about 1.9 to 2.0, athickness of about 700 to 900 Å (70 to 90 nm) is suitable.

In an embodiment, the insulating layer may be deposited on the substrateby methods known in the microelectronics art, such as any form ofchemical vapor deposition (“CVD”) including plasma-enhanced CVD(“PECVD”) or thermal CVD, thermal oxidation, or sputtering. In anotherembodiment, the substrate is coated with a liquid material that underthermal treatment decomposes or reacts with the substrate to form theinsulating layer. In still another embodiment, the substrate isthermally treated in the presence of an oxygen- or nitrogen-containingatmosphere to form an insulating layer. Alternatively, no insulatinglayer is specifically applied to the substrate, but a naturally formingsubstance, such as silicon oxide on a silicon wafer, may function as aninsulating film.

After being deposited, the paste is fired, optionally after first beingdried by exposure to a modestly elevated temperature. Thetime/temperature profile conditions for the firing are typically set soas to, typically involving

In various embodiments, a portion of any insulating layer present,whether specifically applied or naturally occurring, may be removed toenhance electrical contact between the paste composition and theunderlying semiconductor substrate. Preferably, the glass component andlithium-containing additive act to at least partially dissolve theinsulating layer to permit contact to be established.

In an embodiment, the foregoing process can be used to fabricate aphotovoltaic cell. One possible sequence of steps is illustrated in FIG.1( a) through 1(f).

FIG. 1( a) shows a p-type substrate 10, which may be single-crystal,multicrystalline, or polycrystalline silicon. Substrate 10 may besliced, for example, from an ingot that has been formed from a pullingor casting process. Surface damage, e.g. from slicing with a wire saw,and contamination may be removed by etching away about 10 to 20 μm ofthe substrate surface using an aqueous alkali solution such as aqueouspotassium hydroxide or aqueous sodium hydroxide, or using a mixture ofhydrofluoric acid and nitric acid. In addition, a step in which thesubstrate is washed with a mixture of hydrochloric acid and hydrogenperoxide may be added to remove heavy metals such as iron adhering tothe substrate surface. Substrate 10 may have a first major surface 12that is textured to reduce light reflection. Texturing may be producedby etching a major surface with an aqueous alkali solution such asaqueous potassium hydroxide or aqueous sodium hydroxide.

In FIG. 1( b), an n-type diffusion layer 20 is formed to create a p-njunction with p-type material below. The n-type diffusion layer 20 canbe formed by any suitable doping process, such as thermal diffusion ofphosphorus (P) provided from phosphorus oxychloride (POCl₃). In theabsence of any particular modifications, the n-type diffusion layer 20is formed over the entire surface of the silicon p-type substrate. Thedepth of the diffusion layer can be varied by controlling the diffusiontemperature and time, and is generally formed in a thickness range ofabout 0.3 to 0.5 microns. The n-type diffusion layer may have a sheetresistivity from several tens of ohms per square up to about 120 ohmsper square or more.

After protecting one surface of the n-type diffusion layer 20 with aresist or the like, the n-type diffusion layer 20 is removed from mostsurfaces by etching so that it remains only on the first major surface12 of substrate 10, as shown in FIG. 1( c). The resist is then removedusing an organic solvent or the like.

Next, as shown in FIG. 1( d), an insulating layer 30, which alsofunctions as an antireflective coating, is formed on the n-typediffusion layer 20. The insulating layer is commonly silicon nitride,but can also be a film of another material, such as SiN_(x):H (i.e., theinsulating film comprises hydrogen for passivation during subsequentprocessing), titanium oxide, silicon oxide, mixed silicon oxide/titaniumoxide, or aluminum oxide. The insulating layer can be in the form of asingle layer or multiple layers.

Next, electrodes are formed on both major surfaces 12, 14 of thesubstrate. As shown in FIG. 1( e), a paste composition 500 of thisinvention is screen-printed on the insulating layer 30 of the firstmajor surface 12 and then dried. For a photovoltaic cell, pastecomposition 500 is typically applied in a predetermined pattern ofconductive lines extending from one or more bus bars that occupy apredetermined portion of the surface. In addition, aluminum paste 60 andback-side silver paste 70 are screen-printed onto the back side (thesecond major surface 14 of the substrate) and successively dried. Thescreen-printing operations may be carried out in any order. For the sakeof production efficiency, all these pastes are typically processed byco-firing them at a temperature in the range of about 700° C. to about975° C. for a period of from several seconds to several tens of minutesin air or an oxygen-containing atmosphere. An infrared-heated beltfurnace is conveniently used for high throughput.

As shown in FIG. 1( f), the firing causes the depicted paste composition500 on the front side to sinter and penetrate through the insulatinglayer 30, thereby achieving electrical contact with the n-type diffusionlayer 20, a condition known as “fire through.” This fired-through state,i.e., the extent to which the paste reacts with, and passes through, theinsulating layer 30, depends on the quality and thickness of theinsulating layer 30, the composition of the paste, and on the firingconditions. Firing thus converts paste 500 into electrode 501, as shownin FIG. 1( f).

The firing further causes aluminum to diffuse from the back-sidealuminum paste into the silicon substrate, thereby forming a p+ layer40, containing a high concentration of aluminum dopant. This layer isgenerally called the back surface field (BSF) layer, and helps toimprove the energy conversion efficiency of the solar cell. Firingconverts the dried aluminum paste 60 to an aluminum back electrode 61.The back-side silver paste 70 is fired at the same time, becoming asilver or silver/aluminum back electrode 71. During firing, the boundarybetween the back-side aluminum and the back-side silver assumes thestate of an alloy, thereby achieving electrical connection. Most areasof the back electrode are occupied by the aluminum electrode, owing inpart to the need to form a p+ layer 40. Since there is no need forincoming light to penetrate the back side, substantially the entiresurface may be covered. At the same time, because soldering to analuminum electrode is unfeasible, a silver or silver/aluminum backelectrode is formed on limited areas of the back side as an electrode topermit soldered attachment of interconnecting copper ribbon or the like.

The glass material in the paste composition is preferably selected forits capability to rapidly digest the insulating layer. For example, thepaste composition could contain first and second glass components. Thesecond glass component can in such case be designed to slowly blend withthe first glass component while retarding the chemical activity. Astopping condition may result, such that the insulating layer is atleast partially removed but without attacking the underlying emitterdiffused region, which potentially would shunt the device, were thecorrosive action the paste composition is preferably selected for itscapability to rapidly digest the insulating layer. For example, thepaste composition could contain first and second glass components. Thesecond glass component can in such case be designed to slowly blend withthe first glass component while retarding the chemical activity. Astopping condition may result, such that the insulating layer is atleast partially removed but without attacking the underlying emitterdiffused region, which potentially would shunt the device, were thecorrosive action to proceed unchecked. Such a glass component may becharacterized as having a sufficiently high viscosity to provide astable manufacturing window to remove insulating layers without damageto the diffused p-n junction region of the semiconductor substrate.Ideally, the firing process results in a substantially complete removalof the insulating layer without further combination with the underlyingSi substrate.

While the present invention is not limited by any particular theory ofoperation, it is believed that, upon firing, the presence of the lithiumcomponent in the present paste composition promotes etching of theinsulating layer, which in turn permits the formation of alow-resistance, front-side electrical contact between the metal(s) ofthe composition and the underlying substrate.

The nature of the fired-through state, i.e., the extent to which thepresent paste composition formed as electrode 500 melts and passesthrough the insulating layer to form electrical contact with thesubstrate after firing, depends on the quality and thickness of theinsulating layer, the composition of the layer and the electrode paste,and the firing conditions. A high-quality fired-through state isbelieved to be an important factor in obtaining high conversionefficiency in a photovoltaic cell.

It will be understood that the present paste composition and process mayalso be used to form electrodes, including a front-side electrode, of aphotovoltaic cell in which the p- and n-type layers are reversed fromthe construction shown in FIG. 1, so that the substrate is n-type and ap-type material is formed on the front side.

In yet another embodiment, this invention provides a semiconductordevice that comprises a semiconductor substrate having a first majorsurface; an insulating layer optionally present on the first majorsurface of the substrate; and, disposed on the first major surface, aconductive electrode pattern having a preselected configuration andformed by firing a paste composition as described above.

A semiconductor device fabricated as described above may be incorporatedinto a photovoltaic cell. In another embodiment, this invention thusprovides a photovoltaic cell array that includes a plurality of thesemiconductor devices as described, and made as described, herein.

In an embodiment, the processing of photovoltaic device cells utilizesnitrogen, forming gas, or other inert gas firing of the prepared cells.

In an embodiment, the structure may not include an insulating film thathas been applied, but instead may contain a naturally forming substance,such as silicon oxide, which may function as an insulating film.

In an aspect of this embodiment, the glass frits or fluxes arelead-free.

An embodiment of the invention relates to semiconductor device formed bya process described herein. An embodiment of the invention relates to asolar cell including an electrode, which includes a metal powder and oneor more glass frits or fluxes, wherein the glass frits or fluxes arelead-free.

EXAMPLES

The operation and effects of certain embodiments of the presentinvention may be more fully appreciated from a series of examplesdescribed below. The embodiments on which these examples are based arerepresentative only, and the selection of those embodiments toillustrate aspects of the invention does not indicate that materials,components, reactants, conditions, techniques and/or configurations notdescribed in the examples are not suitable for use herein, or thatsubject matter not described in the examples is excluded from the scopeof the appended claims and equivalents thereof. The significance of theexamples is better understood by comparing the results obtainedtherefrom with the results obtained from certain trial runs that aredesigned to serve as controlled experiments and provide a basis for suchcomparison since they do not contain a lithium component in theconductive paste.

Paste Preparation

With the exception of the metal component, all inorganic ingredientsused in the following examples (i.e., glass component andlithium-containing additive component) were ball milled in separatesteps in a polyethylene container with zirconia balls and an appropriatesolvent until the median particle size (D₅₀) was in the range of 0.5-0.7μm. The glass components were lead-free glass frits as described below.Lithium oxide was supplied by Alfa Aesar (#41832, 99.5%); lithiumcarbonate was supplied by Sigma-Aldrich (#431559, 99.99%); and lithiumfluoride was supplied by Sigma-Aldrich (#203645, 99.98%).

All the organic ingredients, including solvents, vehicles, surfactants,binders, and viscosity modifiers were put into a Thinky (Thinky USA,Inc, Laguna Hills, Calif.) mixing jar and Thinky mixed at 2000 RPM for 2to 4 minutes until well blended. All inorganic ingredients were then putinto a glass jar and tumble-mixed for 15 minutes. One third of inorganicingredients were then added to the Thinky jar containing the organicingredients and Thinky-mixed for 1 minute at 2000 RPM. This was repeateduntil all inorganics were added and mixed. The paste was cooled and theviscosity was adjusted to between 200 and 500 Pa-s by adding solvent andThinky mixing for 1 minute at 2000 RPM. This step was repeated until thecorrect viscosity was achieved. The paste was then roll milled at a 1mil gap for 3 passes at zero pressure and 3 passes at 75 PSI using athree-roll mill (Charles Ross & Son Co., Hauppauge, N.Y.).

The degree of dispersion was measured by the fineness of grind (FOG),using test equipment in accordance with ASTM Standard Test Method D1210-05, which is promulgated by ASTM International, West Conshohocken,Pa., and is incorporated herein by reference. In an embodiment, thelargest particles detected in the present paste composition using theFOG test may be approximately 20 μm in size, and the median particlesize may be about 10 μm.

After a 24 hour holding period, to ensure the paste composition hadrheological characteristics suitable for screen printing, its viscositywas measured and adjusted, if necessary, to between 200 and 320 Pa-swith the addition of solvent and Thinky mixing. Viscosity was determinedusing a Brookfield viscometer (Brookfield Inc., Middleboro, Mass.) witha #14 spindle and a #6 cup. Viscosity values were taken after 3 minutesat 10 RPM.

Photovoltaic Cell Fabrication

The performance of conductive pastes of the invention and comparativeexamples was evaluated using photovoltaic cells constructed using 160micron thick Q.Cell (Q-Cells SE, OT Thalheim, Germany) multicrystallinesilicon wafers with a 65 ohm/sq phosphorus-doped emitter layer preparedby a POCl₃diffusion process. As supplied, the wafers had a texturedsurface formed by an acid-etching treatment. A 70-nm thick SiN_(x)antireflective coating had been applied to the front-side major surfaceusing a PECVD process. For convenience, the fabrication and electricaltesting were carried out using “cut down” test wafers, i.e. 28 mm×28 mmwafers diced from 156 mm×156 mm starting wafers using a diamond waferingsaw. The test wafers were screen printed using an AMI-Presco (AMI, NorthBranch, N.J.) MSP-485 screen printer, first to form a full ground planeback-side conductor using a conventional Al-containing paste (DuPontPV381), and thereafter to form a bus bar and eleven conductor lines at a0.254 cm pitch on the front surface using the various exemplary pastesherein.

The cells were dried at 150° C. for 20 minutes after printing the backsides and again after printing the front sides. The dried, printed cellswere fired in a BTU International rapid thermal processing, multi-zonebelt furnace. The firing temperatures set forth in the Examples belowwere the furnace set-point temperatures in the hottest furnace zone. Thefurnace set-point temperature was found to be approximately 125° C.greater than the peak wafer temperature actually attained by the cellduring its passage through the furnace. After firing, the conductorlines had a median line width of 120 μm and a mean line height of 15 μm.The median line resistivity was 3.0 μΩ-cm. Performance of “cut-down”cells, such as the present 28×28 mm cells, is known to be impacted byedge effects, which reduce the overall solar cell fill factor (FF) byapproximately 5% from what would have been obtained with full-sizewafers.

Photovoltaic Cell Electrical Measurements

Photovoltaic cell performance was measured using a ST-1000, Telecom STVCo. IV tester at 25° C.±1.0° C. The Xe arc lamp in the IV testersimulated the sunlight and irradiated the front surface of the cell witha known intensity. The tester used a four-contact method to measurecurrent (I) and voltage (V) at approximately 400 load resistancesettings to determine the cell's I-V curve. Photovoltaic cell efficiency(Eff), fill factor (FF), and series resistance (R_(s)) were calculatedfrom the I-V curve. High values of Eff and FF and a low value of R_(s)are desired for a photovoltaic cell. R_(s) is known to be especiallyaffected by contact resistance (ρ_(c)) and conductor line resistance.Since conductor line resistances were nominally equivalent for thevarious samples (3.0 μΩ-cm), the differences in Rs were primarily due toρ_(c). Ideality factor was determined using the Suns-VOC technique.Ideality factor data are preferably obtained at 0.1 sun irradiance,which is believed to provide a more sensitive indication of diodequality and a more effective measure of p-n junction damage thancomparable data taken at a 1.0 sun irradiance level. A low idealityfactor is desirable.

Efficiency, fill factor, series resistance, and ideality factor valueswere obtained for photovoltaic cells having front-side electrodesprovided from paste compositions having Li-containing additives.Corresponding data were also obtained for cells having front-sideelectrodes made with paste not having Li-containing additives. For eachcondition shown, multiple cells were fabricated and tested. Theperformance values listed at each condition represent the median of thedata measured for these cells.

Comparative Example A

A paste sample was formulated with a composition in which the glass fritwas loaded to 7 wt. % of the inorganics and had the following nominalcomposition, as stated in weight percentage based on the total glassfrit composition given in Table A:

TABLE A SiO₂ Al₂O₃ Bi₂O₃ BiF₃ ZrO₂ ZnO P₂O₅ 11.98 2.75 42.25 17.67 1.5420.28 3.54The silver concentration was adjusted to provide an inorganic solidslevel of 88% in the paste.

The organic medium used had the nominal composition listed in Table Ibelow, the weight percentages based on the total organics present.

TABLE I Organic components of the paste composition Component Wt. %2,2,4-trimethyl-1,3-pentanediol monoisobutyrate 46.42 Ethyl Cellulose(50-52% ethoxyl) 1.17 Ethyl Cellulose (48-50% ethoxyl) 0.33N-tallow-1,3-diaminopropane dioleate 8.33 Hydrogenated castor oil 4.17Pentaerythritol tetraester of perhydroabietic acid 10.42 Dimethyladipate 26.25 Dimethyl glutarate 2.92

A total of 25 samples were screen printed using this paste. Five ofthese samples were fired at each of the following peak temperatures:870, 890, 910, 930 and 950° C. Optimal performance was seen in thesamples fired at a peak firing temperature of 890° C., which had a meanefficiency of 13.78%.

Example 1

The experiment described in Comparative Example A was repeated with thesame organic medium, but with the inclusion of a Li₂CO₃ additive at alevel of 0.6 wt. %, based on the total paste composition. The silverconcentration was adjusted to maintain an inorganic solids level of 88%.Optimal performance was again seen at a peak firing temperature of 890°C., but the mean efficiency of the five samples increased to 14.37%, aclear improvement of over 0.5% efficiency relative to the properties ofthe Comparative Example A cells.

Comparative Example B

The experiment described in Comparative Example A was repeated with thesame organic medium, but with the frit level reduced to 4% and with noLi-component addition. The silver concentration was adjusted to maintainthe inorganic solids level at 88%. Optimal performance was seen at apeak firing temperature of 950° C., which yielded a mean efficiency of12.76% for the five samples.

Examples 2 and 3

The experiment described in Comparative Example B was repeated with thesame organic medium and frit at 1% and 6%, but with Li₂CO₃ addition at0.2 wt. %. In both cases, the silver concentration was adjusted to keepthe inorganic solids level at 88%. With 1% frit, the optimal performancewas at a peak firing temperature of 950° C. and the mean efficiency ofthe five samples was 13.8%, a clear improvement of over 1% efficiencyrelative to the Comparative Example B. With 6% frit, the optimalperformance was at a peak firing temperature of 910° C. and the meanefficiency of the five samples was 14.3%, a clear improvement of over1.5% efficiency relative to the Comparative Example B.

Example 4

A statistically-designed experiment utilizing 12 different compositionswas carried out to determine the optimal content of frit andLi-containing additive. The compositions employed the same frit used inComparative Example A at levels varying from 1 to 9% and with Li₂CO₃addition at levels varying from 0 to 1.6%. In each composition, thesilver concentration was adjusted to keep the inorganic solids level at88%. Statistical analysis of the results using Minitab 15 (Minitab Inc.,State College, Pa.) predicted an optimal efficiency at 0.6% Li₂CO₃ andabout 8% frit.

Comparative Example C

A paste sample was formulated with the same organic medium and silverused in Comparative Example A. The glass frit was loaded to 2% of theoverall paste composition and had the following nominal compositiongiven in Table B:

TABLE B SiO₂ Al₂O₃ B₂O₃ Bi₂O₃ TiO₂ ZrO₂ Na₂O Li₂O 21.92 0.28 3.84 642.01 4.81 1.64 1.5The silver concentration was adjusted to keep the inorganic solids levelat 88%. The samples were screen printed and fired at peak temperaturesof 840, 865, 890, 915 and 940° C. Optimal performance was at a peakfiring temperature of 940° C. and the mean efficiency of the samples was1.09%, which is too low for a photovoltaic cell to be commerciallydesirable.

Examples 5 and 6

The experiment described in Comparative Example C was repeated, but witha frit loading of 2 wt. % and with inclusion of Li₂CO₃ at levels of 0.4and 1.2 wt. %. Again, the silver concentration was adjusted to keep theinorganic solids level at 88%. Optimal performance for the 0.4% samplewas at a peak firing temperature of 890° C. and the median efficiency ofthe five samples was 14.15%, a large improvement of over the ComparativeExample C and also an improvement over Example A. Optimal performancefor the 1.2% sample was at a peak firing temperature of 850° C. and themedian efficiency of the five samples was 10.81%. This value issubstantially higher than obtained for Comparative Example C. It is thusregarded that the there will be an optimal concentration of Li₂CO₃ forany given frit composition, but the addition of discrete Li₂CO₃ to thepresent paste composition still permits the construction of photovoltaiccells having properties that are superior to those constructed withalternative paste compositions that lack Li₂CO₃ or other discreteLi-containing additives used herein.

Examples 7-10

Paste samples were formulated with the same organic medium and silverused in Comparative Example A. The glass frit was loaded to 2% of theoverall paste composition and had the following nominal compositiongiven in Table C:

TABLE C SiO₂ Al₂O₃ ZrO₂ B₂O₃ Na₂O Li₂O Bi₂O₃ 1.86 0.34 0.27 12.95 0.411.59 82.57

Paste composition samples were prepared with discrete Li₂CO₃ additivesas shown in Table II below. The silver concentration was adjusted tokeep the inorganic solids level at 88%. The samples were screen printedonto Si wafers and fired at peak temperatures of 860, 875, 890, and 905°C. Electrical properties of the resulting PV cells were tested asdescribed above. Data for the cells at the optimal firing temperature isset forth in Table II.

TABLE II Processing Conditions and Properties of Photovoltaic Cells FillSeries Ex. Wt % Pk. Temp. Factor Efficiency Resistance Ideality No.Additive (° C.) (%) (%) (Ω) Factor 7 0.27 860 68.7 12.57 0.235 3.7 80.41 875 70.3 13.0 0.237 3.1 9 0.82 890 67.7 12.1 0.239 3.7 10 1.76 90564.2 11.75 0.268 3.9

The efficiency values of the photovoltaic cells shown in Table IIcompare favorably with values exhibited in Comparative Examples A, B,and C. The other characteristics shown in Table II also indicate cellsthat are well suited for their intended use.

What is claimed is:
 1. A paste composition comprising an inorganic solidportion comprising: (a) about 75% to about 99% by weight based on solidsof a source of electrically conductive metal; (b) about 0.1% to about10% by weight based on solids of a substantially lead-free glasscomponent comprising: 1-25 wt. % of SiO₂; 0.1-3 wt. % of Al₂O₃; 50-85wt. % of at least one of bismuth oxide and bismuth fluoride, the amountof bismuth oxide being at least 10 wt. % and the amount of bismuthfluoride being at most 50 wt. %; and at least 1 wt. % of at least one ofTiO₂, ZrO₂, Li₂O, ZnO, P₂O₅, LiF, NaF, KF, K₂O, V₂O₅, GeO₂, TeO₂, CeO₂,Gd₂O₃, or a mixture thereof; wherein the weight percentages are based onthe total glass component; and (c) about 0.1 to about 5% by weight basedon solids of a discrete lithium-containing additive; wherein theinorganic solid portion is dispersed in an organic medium and the pastecomposition is substantially Pb-free.
 2. The paste composition of claim1, wherein the lithium-containing additive is at least one of lithiumoxide, lithium hydroxide, a lithium salt of an inorganic or organicacid, or a mixture thereof.
 3. The paste composition of claim 2, whereinthe lithium-containing additive is LiF.
 4. The paste composition ofclaim 2, wherein the lithium-containing additive is Li₂CO₃.
 5. The pastecomposition of claim 1, wherein the substantially lead-free glasscomponent comprises 8-25 wt. %, based on the total glass component. 6.The paste composition of claim 1, wherein the electrically conductivemetal is at least one of gold, silver, copper, nickel, palladium, or analloy or a mixture thereof.
 7. The paste composition of claim 6, whereinthe electrically conductive metal is silver.
 8. The paste composition ofclaim 1, wherein the source of electrically conductive metal is finelydivided silver particles.
 9. An article comprising: (a) a semiconductorsubstrate having a first major surface; and (b) a deposit of thesubstantially lead-free paste composition of claim 1 on a preselectedportion of the first major surface of the semiconductor substrate. 10.The article of claim 9, wherein an insulating layer is present on thefirst major surface of the semiconductor substrate and the pastecomposition is deposited on the insulating layer.
 11. The article ofclaim 10, wherein the paste composition has been fired to remove theorganic medium and form an electrode that has electrical contact withthe semiconductor substrate.
 12. The article of claim 10, wherein thesemiconductor substrate is a silicon wafer.
 13. A process comprising:(a) providing a semiconductor substrate having a first major surface;(b) applying the substantially Pb-free paste composition of claim 1 ontoa preselected portion of the first major surface, and (c) firing thesubstrate and the paste composition, whereby the organic medium of thepaste composition is removed and an electrode is formed havingelectrical contact with the semiconductor substrate.
 14. The process ofclaim 13, wherein an insulating layer is disposed on the first majorsurface and the paste composition is applied over the insulating layer.15. The process of claim 14, wherein the insulating layer comprises atleast one of aluminum oxide, titanium oxide, silicon nitride, SiN_(x):H,silicon oxide, and silicon oxide/titanium oxide.
 16. The process ofclaim 14, wherein the insulating layer is a naturally occurring layer.17. The process of claim 13, wherein the paste composition is appliedonto the first major surface in a preselected pattern.
 18. The processof claim 13, wherein the firing is carried out in air or anoxygen-containing atmosphere.
 19. The process of claim 13, wherein theelectrically conductive metal is finely divided silver present in anamount ranging from about 75% to 99% by weight based on the inorganicsolids.